August 24, 2012

expert reaction to adolescent cannabis users and neuropsychological health

A large, long term study in PNAS showed persistent cannabis use may be associated with neuropsychological decline, particularly among adolescent users.

Prof Robin Murray, Professor of Psychiatric Research, Institute of Psychiatry, Kings College London, said:

“This is a very impressive paper. The Dunedin sample is probably the most intensively studied cohort in the world and therefore the data are very good. The researchers, who I know well, are among the best epidemiologists in the world. Therefore, although one should never be convinced by a single study, I take the findings very seriously.

“We have known for some time that heavy use of cannabis increases risk of schizophrenia-like psychoses but this remains a relatively rare outcome so it’s not so important from a public health point of view.

“There are far fewer studies on its effect on minor psychiatric illness or on everyday life. However, there are a lot of clinical and educational anecdotal reports that cannabis users tend to be less successful in their educational achievement, marriages and occupations. It is of course part of folk-lore among young people that some heavy users of cannabis (my daughter callers them stoners) seem to gradually lose their abilities and end up achieving much less than one would have anticipated. This study provides one explanation as to why this might be the case.

“In a few years, to my surprise, a number of studies have shown that heavy use of cannabis is associated with subtle loss of certain brain volumes e.g. grey matter, hippocampus. It would be very interesting to examine the relationship between brain volume changes and cognitive decline.

“The view that adolescent use is more toxic is a fashionable one (particularly among adults!). It is difficult to totally control for a number of factors. For instance, adolescent users tend to attract a lot of negative consequences (chucked out of school etc) while no-one notices starting use at university. Also early users are more likely to become dependent and of course have more years of use.

“Another important issue, which they don’t address, is the type of cannabis used. We now know that use of high potency types of cannabis (called skunk in the UK) appears to be more likely to cause problems. They don’t address this issue.

“However, these are minor points. I suspect that the findings are true. If and when they are replicated then it will be very important and public education campaigns should be initiated to let people know the risks.”

‘Persistent cannabis users show neuropsychological decline from childhood to midlife’ by Meier et al. , published in PNAS at 8pm UK time on Monday 27th August.